1. Field of the Invention
The present invention generally relates to methods and apparatus for promoting an intervertebral fusion, and more particularly to an apparatus for insertion into a space between adjacent vertebrae to facilitate an intervertebral fusion while maintaining a substantially natural lordosis of the human spine.
2. Description of the Related Art
Intervertebral discs that become degenerated due to various factors such as trauma or aging typically have to be partially or fully removed. Removal of an intervertebral disc can destabilize the spine, making it necessary to replace the vertebral disc to maintain the height of the spine and to fuse the spine. Spinal implants are often used to prevent collapse of the spine. U.S. Ser. No. 08/740,123 filed Oct. 24, 1996 relates to methods and apparatus for facilitating a spinal fusion and is incorporated by reference as if fully set forth herein.
After an intervertebral disc is removed, an implant device is typically inserted between neighboring vertebrae to maintain normal disc spacing and restore spinal stability, thereby facilitating an intervertebral fusion. A conventional implant device disposed between neighboring vertebrae is depicted in FIGS. 1 and 2. The implant device contains a pair of engaging elements 20 that typically contain threading 10 to engage the vertebrae. Prior to inserting the engaging elements, a vertebral drill is typically inserted within the surgical wound to drill into the cortical endplate and remove fibrous and nuclear material. A vertebral tap may then be used to cut threads into the ends of the neighboring vertebrae. The engaging elements tend to be relatively inflexible and substantially undeflectable. The engaging elements are typically packed with bone graft to facilitate a spinal fusion.
Conventional implant devices tend to not maintain the "lordosis" or natural curvature of the lower lumbar spine. As shown in FIG. 1, the implant device contains parallel engaging sides 12 and 13 to contact vertebra 15. It is typically required that the engaging sides be parallel to prevent the fusion cage from slipping from the intervertebral space. The parallel configuration of the fusion cage tends to alter the lordosis of the spine. Such a loss of lordosis may result in an increased risk to other intervertebral discs located adjacent to the fusion level that may degenerate due to the altered force transmission in the spine.
FIG. 2 depicts a front view of the engaging elements 20 of the implant device. The engaging elements are substantially cylindrical and the region of contact between an engaging element and a vertebra is defined by arcuate portion 22. The cylindrical geometry of the engaging elements tends to provide a relatively small area of contact between the fusion cage and the vertebrae. The weight of the spine creates pressure on the vertebrae that is concentrated proximate the arcuate portions. Subsidence or deformation of the cortical layer of the vertebrae tends to result.
U.S. Pat. No. 5,522,899 to Michelson relates to a spinal implant for placement into the spinal disc space to stabilize the spine and participate in a vertebra to vertebra bony fusion. U.S. Pat. No. 5,489,308 to Kuslich et al. relates to an implant for use in spinal stabilization that includes a cylindrical body having external threading and radially disposed openings positioned to chip bone into an interior portion of the body when the implant is installed. The above-mentioned patents are incorporated by reference as if fully set forth herein.
The above-mentioned prior methods and systems inadequately address, among other things, the need to maintain the natural lordosis of the spine. It is therefore desirable that an improved spinal implant be derived for facilitating an intervertebral body fusion.